Oilfield drilling occurs in geological formations having various compositions, permeabilities, porosities, pore fluids, and internal pressures. Weak zones may occur in a formation during drilling due to the formation having a variety of these conditions therein. These weak zones may lead to fluid loss, pressure changes, well cave-iris, etc. The formation of weak zones is detrimental to drilling because they need to be strengthened before drilling work may resume.
Weak zones may occur, for example, when the fracture initiation pressure of one formation is lower than the internal pore pressure of another formation. As another example, increased borehole pressure, created by penetrating one formation, may cause a lower strength formation to fracture. As another example, the fluid pressure gradient in a borehole required to contain formation pore pressure during drilling may exceed the fracture pressure of a weaker formation exposed in a borehole,
Generally, weak zones have been strengthened by pumping a fluid into the weak. zone, letting the fluid cure and develop strength over a period of time. Fluids used in the past include cement, epoxy resins with amine initiators and vinyl toluenes with initiators. The cure time for cement may be as long as 24 hours, delaying oil production which is undesirable, especially for off-shore drilling with high operating costs. Cement's particle based structure may also exhibit poor penetration capabilities in the formation leading to a reduced sealing effect. When using epoxy resins or vinyl toluenes, the cure time may be reduced, but the compositions are toxic, highly corrosive, flammable and pose a health hazard.
Cement, or other fluid compositions used for strengthening weak zones, may also be used in primary cementing operations which fill at least a portion of the annular space between the casing and the formation wall with the fluid. The cement may then be allowed to solidify in the annular space, thereby forming an annular sheath of cement. The cement barrier is desirably impermeable, such that it will prevent the migration of fluid between zones or formations previously penetrated by the wellbore.
Generally, the cement or other strengthening composition is mixed at the surface and pumped downhole at high pressure to fill in the weak zone. Once the composition fills in the weak zones, it is allowed to set or cure, harden within the wellbore over a period of time.